1. Field of the Invention
The present invention relates generally to electronic photography apparatus and, more particularly, to an electronic photography apparatus suitable for use in developing an electrostatic latent image in an electronic photography process or similar process.
2. Description of the Prior Art
In the electrostatic process such as an electronic photograph developing process or similar process, an electrostatic latent image is formed in such a manner that a photoconductive material such as a photoconductor is uniformly electrified (charged) and then selectively illuminated by a light in accordance with an image signal, so that charges on the portion illuminated by the light are extinguished. Alternatively, a dielectric material such as a paper, a plastic film or similar element is electrified by an electrostatic electrode called a multistylus head in response to an image signal, thereby forming an electrostatic latent image. In order to develop this electrostatic latent image, a toner or developer charged to the polarity opposite to that of the electrostatic latent image carrier (photoconductor drum, dielectric film and so on) having the electrostatic latent image is electrostatically deposited on the electrified portion of the photoconductor drum and then developed.
As a developing method, a dry type developing system using a dry developer and a wet type developing system using liquid developer are known. The dry developer is generally formed of very small particle powders. For this reason, if the dry developer is scattered, a problem of environmental disruption occurs. To solve this problem, a developer cartridge in which the dry developer is accommodated should be constructed as a sealed type. Today, most of the developing apparatus are of such a type that an electrostatic latent image carrier that the developing portion are wholly removed and replaced. Therefore, this type of developing apparatus is expensive but the dry developer is excellent in preservation and if the developer is accommodated within the developer cartridge, it is easy to handle.
On the other hand, the liquid developer is formed by dispersing into an insulating liquid powders of colorant such as dye stuff and the like. By a centrifugal pump or the like, the liquid developer is injected from the developer container through the slit of a developing electrode used to charge the electrostatic latent image to the polarity opposite to that of the electrostatic latent image carrier, whereby colored particle powders are electrostatically deposited on the electrostatic latent image carrier. In the conventional developing apparatus, extra liquid developers, which are not deposited on the electrostatic latent image carrier, are returned to and accommodated again within the developer container, rendering the colorant powders in the liquid developers low in concentration. This makes control of the concentration of the developing liquid (liquid developer) difficult. Further, various problems arise such that pollution occurs due to the leakage of liquid developer in the developing process or when the developing apparatus is held or when the liquid developer is exchanged and that the preservation of the liquid developer is difficult because the colored particle in the powders of the liquid developer tend to coagulate and precipitate. The electrostatic process utilizing the liquid developer has a possibility that resolution and gradation of picture thereof will be increased to levels equal to those of silver halide photograph. Therefore, this electrostatic process is expected to be the electrostatic latent image developing system which is suitably applied to a printing apparatus of high image quality such as a video printer used in an electronic still camera or the like.
In regard to the background set forth so far, the assignee of the present application has previously proposed a method of developing an electrostatic latent image (see Japanese Patent Application No. 63-156847). This previously-proposed method can solve various problems, such as that the liquid developer is difficult to handle, the liquid developer tends to be smudged, the maintenance of liquid developer is difficult and the preservation of liquid developer is poor while making effective use of advantages of the electrostatic process using the liquid developer. In other words, this electrostatic latent image developing method is characterized by a developer (i.e. toner) which is formed by a colorant dispersed into an electrostatic insulating organic material that is solid at normal temperature and when heated is changed into liquid, in which an electrostatic latent image is developed by the thus liquefied developer in a wet developing fashion.
FIG. 1. shows a cross-sectional view of a conventional developing apparatus that is described, for example, in Japanese Patent Published Gazette No. 64-6462.
In FIG. 1, reference numeral 30 designates supporting portions 31 a photoconductor film extended between the supporting portions 30, 32 a preserving tank, 33 a developing liquid preserved in this preserving tank 32, 34 a developing roller, 35 a liquid lifting member, 36 an injection opening portion, 37 a developing electrode, 38 a spring and 39 a bias voltage source.
The developing liquid 33 from the preserving tank 32 is lifted by the rotation of the developing roller 34 via the liquid lifting member 35, injected from the injection opening portion 36 and is filled in the space between the developing electrode 37 and the photoconductor film 31 for development. After the development, the developing liquid 33 is returned to the preserving tank 32 and subsequently utilized several times for the development.
Incidentally, in the case of the conventional apparatus described in Japanese Patent No. 63-156847, if the developing electrode is located closer to the photoconductor drum, the developing is more effectively promoted but the amount of the developer (developing liquid) flowing into the developing space is reduced. Thus, the optimum value exists there. Further, if the developing electrode is too close to the photoconductor drum, the developer forms a meniscus (film formed by surface tension) between the photoconductor material and the developing electrode after the supply of developers is stopped, and this waste developer cannot be discharged. If the waste developer becomes dried and solidified on the photoconductor material, the apparatus cannot be cleaned when it is actuated again. Further, a service life of the photoconductor material and the developing electrode is reduced and a trouble occurs in the apparatus when the apparatus is actuated again.
In the case of the conventional apparatus disclosed in Japanese Patent Published Gazette No. 64-6462, since the developing liquid returned to the preservation tank several times, the composition of the developing liquid (developer) is changes due to aging, resulting in an image quality being deteriorated. Furthermore, if the developing liquid is left for a long period of time, the developers are precipitated and a dispersion property of the developer is deteriorated.